1. Field of the Invention
The present invention relates to a thermoplastic material, a process for producing the thermoplastic material and use of the thermoplastic material in a golf ball.
1. Description of the Related Art
Traditional golf ball covers have been comprised of balata or blends of balata with elastomeric or plastic materials. Balata-related covers, often referred to as soft balata covers, are relatively soft and flexible. Upon impact, soft balata covers compress against the surface of the club producing high spin. Consequently, these soft and flexible covers provide an experienced golfer with the ability to apply a spin to control the ball in flight in order to produce a draw or a fade, or a backspin which causes the ball to “bite” or stop abruptly on contact with the green. Moreover, soft balata covers produce a soft “feel” to the low handicap player. Such playability properties as, workability and feel are particularly important in short iron play with low swing speeds and are exploited significantly by relatively skilled players.
Despite all the benefits of balata, balata-related golfball covers are easily cut and/or damaged if hit improperly. Golfballs produced with balata or balata-containing cover compositions therefore have a relatively short lifespan. As a result of this negative property, balata and its synthetic substitutes, trans-polybutadiene and trans-polyisoprene, have been essentially replaced as the cover materials of choice by new cover materials comprising ionomeric resins.
Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability and flight characteristics, various ionomeric resins sold by E.I. du Pont de Nemours and Company (DuPont), under the trade name “Surlyn7” (Surlyn7™), and, more recently, by the ExxonMobil Corporation (ExxonMobil) (see, for example, U.S. Pat. No. 4,911,451), under the trade name “Iotek” (Iotek™), have become the materials of choice for the construction of golf ball covers over traditional balata (trans-polyisoprene, natural or synthetic) rubbers.
Ionomeric resins are generally ionic copolymers of an olefin (such as ethylene) and a metal salt of an unsaturated carboxylic acid (such as acrylic acid, methacrylic acid or maleic acid). Metal cations such as sodium or zinc are used to neutralize some portion of the acidic group in the copolymer resulting in a thermoplastic elastomer exhibiting enhanced properties such as durability for golf ball cover construction over balata. However, some of the advantages gained in increased durability have been offset to some degree by decreases produced in playability. This is because although ionomeric resins are very durable, they tend to be very hard when utilized for golf ball cover construction and, thus, lack the degree of softness required to impart the spin necessary to control the ball in flight. Since the ionomeric resins are harder than balata, the ionomeric resin covers do not compress as much against the face of the club upon impact, thereby producing less spin. In addition, the harder and more durable ionomeric resins lack the feel characteristic associated with the softer balata-related covers.
As a result, while there are many commercial grades of ionomers available both from DuPont and ExxonMobil, with a wide range of properties that vary according to the type and amount of metal cations, molecular weight, composition of the base resin (such as relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcement agents, or the like, a great deal of research continues in order to develop a golf ball cover composition exhibiting not only the improved impact resistance and carrying distance properties produced by the “hard” ionomeric resins, but also the playability (for example, “spin”, “feel” and the like) characteristics previously associated with soft balata-related covers, properties that are still desired by the more skilled golfer.
Consequently, a number of golf balls have been produced to address these needs. The different types of materials utilized to formulate the cores, mantles and covers of these balls dramatically alter the balls' overall characteristics. In addition, multi-layered covers containing one or more ionomeric resins have also been formulated in an attempt to produce a golf ball having the overall distance, playability and durability characteristics desired.
Such formulations are described in U.S. Pat. No. 4,431,193 ('193), where a multi-layered golf ball is produced by initially molding a first cover layer on a spherical core and then adding a second layer. The first layer consists of a hard, high flexural modulus resinous material such as Surlyn7™ 8940, a sodium ion based low acid (less than or equal to 16 weight percent methacrylic acid) ionomeric resin having a flexural modulus of about 51,000 psi. An outer layer of a comparatively soft, low flexural modulus resinous material such Surlyn7™ 9020 is molded over the inner cover layer. Surlyn7™ 9020 is a zinc ion based low acid (10 weight percent methacrylic acid) ionomeric resin having a flexural modulus of about 14,000 psi.
The '193 patent also teaches that the hard, high flexural modulus resin, which comprises the first layer, provides for a gain in coefficient of restitution over the coefficient of restitution of the core. The increase in the coefficient of restitution provides a ball that attains or approaches the maximum initial velocity limit of 255 feet per second, as provided by the United States Golf Association (USGA) rules. The relatively soft, low flexural modulus outer layer provides for the advantageous feel and playing characteristics of a balata covered golf ball.
In various attempts to produce a durable, high spin golf ball, the golfing industry has blended the hard ionomeric resins with a number of softer ionomeric resins. For example, U.S. Pat. Nos. 4,884,814 and 5,120,791 are directed to cover compositions containing blends of hard and soft ionomeric resins. The hard copolymers typically are made from an olefin and an unsaturated carboxylic acid. The soft copolymers are generally made from an olefin, an unsaturated carboxylic acid and an acrylate ester. However, it has been found that golf ball covers formed from hard-soft ionomer blends tend to become scuffed more readily than covers made of a hard ionomeric resin alone. It would be useful to develop a golfball having a combination of softness and durability that is better than the softness-durability combination of a golf ball cover made from a hard-soft ionomer blend.
Most professional golfers and good amateur golfers desire a golf ball that provides distance when hit off a driver, control and stopping ability on full iron shots as well as high spin on short “touch and feel” shots. Many conventional golfballs have undesirable high spin rates on full shots. The excessive spin on full shots is a sacrifice made in order to achieve more spin on the shorter touch shots. It would be beneficial to provide a golf ball that has high spin for touch shots, without generating excessive spin on full shots, while maintaining or improving some of the other properties of the golf ball.